A nitride semiconductor such as GaN, AlN, InN and the like and a material of a mixed crystal thereof may have a wide band gap, so as to be used as a high-power electronic device or a short-wavelength light-emitting device. Among these, research and development has been conducted on the technologies of a Field-Effect Transistor (FET) and especially a High Electron Mobility Transistor (HEMT) as a high-power device (see, for example, Japanese Laid-open Patent Publication No. 2002-359256).
The HEMT using such a nitride semiconductor is used in a high-power and highly-efficient amplifier, a high-power switching device and the like.
In the HEMT using such a nitride semiconductor, an aluminum gallium nitride/gallium nitride (AlGaN/GaN) heterostructure is formed on the substrate, so that the GaN layer thereof may serve as an electron transit layer. Further, the substrate may be formed of sapphire, silicon carbide (SiC), gallium nitride (GaN), silicon (Si) or the like.
Among the nitride semiconductors, for example, GaN has excellent electronic characteristics because of its higher withstand-voltage characteristic due to its higher saturated electron speed and wider band gap. Further, GaN has a wurtzite-type crystal structure, so as to have its polarity in <0001> direction which is parallel to the c-axis.
Further, when the AlGaN/GaN heterostructure is formed, in the AlGa layer, a piezoelectric polarization may be excited due to lattice distortion between AlGaN and GaN. Therefore, highly-concentrated Two-Dimensional Electron Gas (2DEG) may be generated near an interface (a boundary surface) of the channel. As a result, the HEMT using GaN is thought to be promising as a high frequency power device.
In the HEMT using the nitride semiconductor, by using a large and inexpensive silicon substrate as the substrate, the cost may be largely reduced. Accordingly, the HEMT using the nitride semiconductor may be provided in lower cost. The silicon substrate is electrically-conductive. Therefore, when such a silicon substrate is used, a nitride layer having higher insulation properties may be formed on the silicon substrate, and a nitride semiconductor layer such as the electron transit layer may be formed on the nitride layer.
However, due to differences in a lattice constant and a coefficient of thermal expansion between silicon and nitride, a bend or a crack is likely to be formed in the substrate or the nitride semiconductor layer. Therefore, it may be difficult to form a thick nitride layer having higher insulation properties. As a result, leakage current in the substrate-gate direction is likely to be increased, and it is difficult to ensure sufficient withstand voltage in the vertical direction (i.e., the thickness direction of the substrate).
As a method of forming a thick nitride layer on the silicon substrate while controlling the generation of the bend or crack, there is a known technique in which a Strained Layer Superlattice (SLS) buffer layer is formed where a GaN-based thin film and an AlN-based thin film are alternately formed in multiple cycles (see, for example, Japanese Laid-open Patent Publication Nos. 2012-23314 and 2007-67077).
In the SLS buffer layer, a thick nitride layer may be formed, while controlling the generation of the bend or crack due to the difference in the lattice constant during the formation of the films, by forming the GaN-based thin film and the AlN-based thin film, each being included in the Superlattice and having a thickness less than or equal to its critical film thickness.
Further, in the SLS buffer layer, by containing a large compression strain in the films of the SLS buffer layer, another large compression strain may be generated across the entity of the nitride layer while the temperature is decreased after the films are formed. As described above, by forming the SLS buffer layer, the thickness of the AlN layer having a wider band gap and higher insulation properties may be increased. As a result, the withstand voltage in the vertical direction may be improved.